U.S. patent number 5,677,511 [Application Number 08/406,450] was granted by the patent office on 1997-10-14 for overmolded pc board with esd protection and emi suppression.
This patent grant is currently assigned to National SEmiconductor Corporation. Invention is credited to Michael William Patterson, Carl James Taylor.
United States Patent |
5,677,511 |
Taylor , et al. |
October 14, 1997 |
**Please see images for:
( Certificate of Correction ) ** |
Overmolded PC board with ESD protection and EMI suppression
Abstract
An apparatus directed to portable peripheral cards is disclosed
which provides protection against electro-static discharge and
electro-magnetic interference. Furthermore, this apparatus provides
a solid housing which affords a strong protective structure for the
PC board and also protects the ICs housed inside the peripheral
card from being easily accessed.
Inventors: |
Taylor; Carl James (Morgan
Hill, CA), Patterson; Michael William (Pleasanton, CA) |
Assignee: |
National SEmiconductor
Corporation (Santa Clara, CA)
|
Family
ID: |
23608047 |
Appl.
No.: |
08/406,450 |
Filed: |
March 20, 1995 |
Current U.S.
Class: |
174/527; 174/541;
174/546; 174/560; 257/788; 257/789; 361/737; 361/818 |
Current CPC
Class: |
G06F
13/409 (20130101); H05K 3/284 (20130101); H05K
5/0208 (20130101); H05K 5/0269 (20130101); H01L
2224/48091 (20130101); H01L 2224/48091 (20130101); H01L
2924/00014 (20130101) |
Current International
Class: |
G06F
13/40 (20060101); H05K 5/02 (20060101); H05K
3/28 (20060101); H01L 023/28 (); H01L 023/29 ();
H05K 001/14 (); H05K 009/00 () |
Field of
Search: |
;174/52.1-52.4
;257/787-790,795 ;361/737-739,749,760,816,820,818 ;439/607-610 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1116756 |
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Nov 1961 |
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DE |
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4341896 |
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Apr 1993 |
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JP |
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5201184 |
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Nov 1993 |
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JP |
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96/03021 |
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Feb 1996 |
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WO |
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Primary Examiner: Ledynh; Bot L.
Attorney, Agent or Firm: Skjerven, Morrill, MacPherson,
Franklin & Friel LLP Winters; Paul J. Kwok; Edward C.
Claims
We claim:
1. A portable peripheral card comprising:
a printed circuit board (PC board) having an active region where
one or more electrical components are mounted;
an electrical connector attached to the PC board, whereby external
devices electrically access said one or more electronic components
via said electrical connector;
a solid package encapsulating the PC board yet leaving a portion of
said electrical connector exposed, said solid package being
composed of a conductive segment and a non-conductive segment,
wherein said non-conductive segment encapsulates said active region
of the PC board, and said conductive segment encapsulates a
remaining outer perimeter of the PC board surrounding said active
region; and
a conductive layer overlaying said solid package such that said
conductive layer substantially covers the area over said
non-conductive segment and further extends over said conductive
segment.
2. A portable peripheral card as recited in claim 1 wherein said
conductive segment is from metal and said non-conductive segment is
from polymer.
3. A portable peripheral card as recited in claim 2 wherein said
polymer is thermoset plastic material.
4. A portable peripheral card as recited in claim 2 wherein said
metal is pre-fabricated and is clamped to the PC board.
5. A portable peripheral card as recited in claim 1 wherein said
conductive segment is filled polymer and said non-conductive
segment is polymer.
6. A portable peripheral card as recited in claim 5 wherein a
conductive trace travels around said outer perimeter of the PC
board and electrically contacts said electrical connector.
7. A portable peripheral card as recited in claim 6 wherein said
conductive trace is a ground trace.
8. A portable peripheral card as recited in claim 1 wherein said
conductive segment is from thermoplastic material and said
non-conductive segment is from thermoset plastic material.
9. A portable peripheral card as recited in claim 1 wherein said
conductive layer is secured to said solid package via a conductive
adhesive.
10. A portable peripheral card as recited in claim 9 wherein said
conductive adhesive is z-axis adhesive.
11. A portable peripheral card as recited in claim 1 wherein said
conductive layer is a sheet of metal.
12. A portable peripheral card as recited in claim 1 wherein said
peripheral card is a PCMCIA card.
13. A portable peripheral card comprising:
a printed circuit board (PC board) having an active region where
one or more electrical components are mounted;
an electrical connector attached to the PC board, whereby external
devices electrically access said one or more electronic components
via said electrical connector;
a solid package encapsulating the PC board yet leaving a portion of
said electrical connector exposed, said solid package being
composed of a conductive segment and a non-conductive segment,
wherein said non-conductive segment encapsulates said active region
of the PC board, and said conductive segment encapsulates a
remaining outer perimeter of the PC board surrounding said active
region; and
a conductive trace that travels around said outer perimeter of the
PC board and electrically contacts said electrical connector.
14. A portable peripheral card as recited in claim 13 wherein said
conductive trace is a ground trace.
15. A portable peripheral card as recited in claim 14 further
comprising a conductive layer overlaying said solid package such
that said conductive layer substantially covers the area over said
non-conductive segment and further extends over said conductive
segment.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to the application entitled "Removable
Computer Peripheral Cards Having a Solid One-piece Housing and
Method of Manufacturing the Same", filing date Jul. 15, 1994,
invented by Hem P. Takiar and Michael W. Patterson, Ser. No.
08/275985.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to the packaging of
electronic assemblies. More particularly, it is directed to the
manufacture of peripheral cards such as PCMCIA cards having
overmolded housing and protection against both electro-static
discharge (ESD) and electro-magnetic interference (EMI).
2. Description of the Related Art
The technological progress in personal computers has led to high
demand for expansion boards to enhance their functions. Originally,
these boards were unprotected printed circuit boards (PC boards)
having edge connectors and carrying a variety of circuit components
soldered thereto. These boards were installed inside the computer
and therefore required the opening of the case of the computer.
However, the advent of the laptop personal computers along with the
rapid progress in miniaturization of integrated circuits have
brought about the development of the portable peripheral card. A
particular type of peripheral card known as PCMCIA card has emerged
as the industry standard. Many of the current laptop and notebook
computers contain built-in support for PCMCIA cards. PCMCIA cards
are inserted in external slots of a laptop or notebook computer
without the inconvenience of opening the computer.
Even though originally developed for memory expansion cards (the
acronym "PCMCIA" stands for "Personal Computer Memory Card
International Association"), the PCMCIA standard has since seen
wide adoption in a variety of applications, such as FLASH memory
cards, hard disks, FAX/modem, security cards, etc. In many of the
applications, such as security cards where the card is carried with
the card holder in much the same way as a credit card,
susceptibility to ESD is of primary concern. Furthermore, as these
peripheral cards are packed in with more complex circuitries,
adequate shielding against EMI also becomes of great
importance.
Electro-static charge may build-up on any surface area of a
peripheral card. If this charge is not properly discharged, it can
cause damage to both the electronic components inside the card and
the computer to which the card is coupled. Therefore, an effective
discharge path is needed from any surface area of the card.
To adequately shield the peripheral card against radiating or
receiving EMI, the shield must have a low enough resistivity so
that no electro-magnetic energy can penetrate. Therefore, the
shield must have a combination of adequate thickness and
conductivity. For example, a relatively thin continuous metal
shield can provide adequate shielding at all frequencies. A polymer
filled with conductive material, hereinafter referred to as filled
polymer, on the other hand is likely to provide poor shielding at
particular frequencies. Furthermore, the shield needs to provide a
continuous conductive surface area to ensure uninterrupted surface
current flow. Any discontinuities in the shield leads to what is
called a slot antenna, through which EMI can penetrate.
A number of packaging techniques are available in constructing
peripheral cards such as PCMCIA cards, some of which have been
modified to provide limited protection against ESD and EMI.
One common packaging technique has been the use of sheet metal
housing. FIG. 1 depicts a PCMCIA card constructed in accordance
with this technique. A PC board 50 is electrically coupled to a
female PCMCIA edge connector 51. The PC board 50 fits in a groove
or shelf in a plastic rim 52 surrounding the PC board 50. Sheet
metal jackets 53 are attached to the top and bottom of the assembly
to form the complete PCMCIA card. Since the metal jackets 53 are
usually fairly flexible, sheets of plastic insulators 55 are placed
between the PC board 50 and the metal jackets 53. The plastic
insulators 55 preclude the PC board 50 from coming into electrical
contact with the metal jackets 53, thereby preventing electrical
shorts.
ESD protection is provided by electrically coupling the metal
jackets 53 to a ground trace on the PC board via conductive clips.
The conductive clips are attached to ground pads on the PC board.
These clips are raised from the PC board surface to make electrical
contact with the metal jackets 53. Therefore, an electrostatic
discharge path is provided from the metal jackets 53 to the ground
circuit on the PC board via the conductive clips.
In protecting against EMI, the two metal jackets 53 provide
adequate shielding in the surface area. However, where the two
metal jackets 53 are butted together a slot antenna is formed
through which EMI can penetrate. Several techniques have been used
to ensure full electrical contact between the metal jackets 53
however, none have proven effective. Additional conductive clips
are used to short the two halves of the case together to reduce the
length of the slot antenna, thereby reducing the magnitude of the
EMI problem.
The above packaging technique however, possesses a number of
drawbacks. First, the manufacturing process must align and attach
several parts: the PC board, the edge connector, the plastic rim,
the conductive clips, the sheets of plastic insulators and the
metal jackets. This multistage assembly process is time consuming
and labor intensive and therefore quite costly. Second, the sheet
metal jackets are generally fairly flexible implying a relatively
weak external protective structure for the PC board. Third, the
metal jackets do not fully protect against EMI since a perfect
electrical contact may not be obtained between the two metal
jackets, and the conductive clips will not fully eliminate EMI.
Fourth, given the ease with which the metal jackets can be pried
open, the ICs housed inside are easily accessible. This is of
concern in applications such as security cards, where the ICs carry
proprietary data.
Another method of packaging Peripheral cards is disclosed in the
aforementioned patent document of Takiar et al., incorporated
herein by reference. This method provides a solid one-piece
injection molded package, housing the PC board. This method
significantly simplifies the manufacturing process, thereby
reducing the cost. Furthermore, the solid one-piece package
provides a stronger external protective structure for the PC board
as well as an added level of security for the ICs housed inside the
card (i.e. the package can not easily be pried open).
Even though this method removes some of the drawbacks of the prior
art, it however introduces others. This method requires the use of
non-conductive molding material such as thermoset plastic, and such
non-conductive housing renders the peripheral card and the ICs
inside susceptible to both ESD and EMI.
From the above, there has been a need for a method of manufacturing
peripheral cards such that the resulting card would have a solid
housing replacing the several parts, while ESD and EMI protection
are not compromised.
SUMMARY
In accordance with the present invention, a portable peripheral
card, such as a PCMCIA card, having an overmolded housing package
is disclosed which will overcome the drawbacks of the prior
art.
In one aspect of the invention, the peripheral card comprises a
printed circuit board (PC board) having an active region where one
or more electronic components are mounted, and an outer peripheral
region surrounding the active region. An edge connector is attached
to the PC board to enable communication between external devices
and the electronic components. A solid overmolded package is
provided which encapsulates the printed circuit board and the
electronic components, yet exposes a portion of the edge connector
to facilitate electrical connections between external devices and
the PC board.
The solid package is composed of a conductive segment and a
non-conductive segment. The non-conductive segment encapsulates the
active region of the PC board and the conductive segment
encapsulates the outer periphery surrounding the active region. A
conductive layer is secured to each of the top and bottom surfaces
of the housing via conductive adhesive. Each conductive layer
covers the entire surface area over the active region and extends
over a portion of the outer periphery region.
The peripheral card as described above protects the PC board and
the electronic components from damage caused by ESD, by providing
an effective discharge path for electro-static charge built up
anywhere on the surface of the peripheral card. The discharge path
is from the conductive layer to the conductive segment of the
package via the conductive adhesive, and from the conductive
segment to the ground terminal of the edge connector via a
conductive trace on the PC board.
The peripheral card described above also provides effective
shielding against electro-magnetic interference. The conductive
layer provides shielding in the surface area while the conductive
segments in conjunction with the ground trace provide shielding in
the periphery area. Therefore, the PC board is surrounded by
continuous conductive material, and thus is shielded against
EMI.
One feature of this invention is that it provides an effective
discharge path for electro-static charge build-up anywhere on the
surface of the peripheral card.
Another feature of this invention is that it provides adequate
shielding around the peripheral card against radiating or receiving
EMI.
Yet another feature of this invention is that it provides a solid
package for housing the PC board, thereby providing a strong
protective structure for the PC board as well as protecting the ICs
housed inside the peripheral card from being easily accessible.
Yet another feature of this invention is that the above features
are achieved while minimizing the manufacturing cost of the
peripheral cards.
Yet another feature of this invention is that the above features
are achieved within the constraints set by the industry on the
physical dimensions of the peripheral card as well as on its other
aspects.
These and other features and advantages of the present invention
will become more apparent from the following description and the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded view of a conventional PCMCIA peripheral
card.
FIG. 2 is a plan view of a peripheral card made in accordance with
the present invention, wherein the outer periphery of the PC board
containing the ground trace is shown as being exposed.
FIG. 3 is a sectional view of the peripheral card in accordance
with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A first embodiment according to the present invention is shown in
FIGS. 2 and 3. As shown in FIG. 3, one or more electronic
components 15 such as packaged integrated circuits, resistors and
capacitors are mounted on a printed circuit board (PC board) 11.
The area of the PC board 11 where the electronic components 15 are
located will be referred to as the active regions, and is shown in
FIG. 2 as regions 13. An edge connector 10 is electrically coupled
to the PC board 11. The edge connector 10 provides the means
through which external devices electrically access the electronic
components 15. As shown in FIG. 2, a ground trace 12 travels around
the outer perimeter 18 of the PC board 11 surrounding the active
regions 13. In FIG. 3 the ground trace 12 is shown as being raised
from the PC board surface for pictorial clarity, where in reality
it is embedded in the PC board 11.
Through two consecutive molding processes the PC board 11 is
completely encapsulated with packaging material while leaving a
portion of the electrical connector 10 exposed. For the first
molding process, a single cavity molding tool is developed which
encapsulates only the active regions 13 of the PC board 11, leaving
the outer perimeter 18 exposed. The first molding process is
carried out by first positioning the PC board 11 in the mold, and
subsequently injecting a non-conductive packaging material into the
mold, encapsulating the active regions 13 of the PC board 11. As a
result, non-conductive regions 19 are formed over the active
regions 13. The non-conductive packaging material used is
preferably thermoset plastic material, although other suitable
non-conductive material may be used.
To carry out the second molding process, another single cavity
molding tool is developed for encapsulating only the exposed outer
perimeter 18 of the PC board 11. As in the first molding process,
the PC board 11 is positioned in the second mold, however, a
conductive packaging material is injected into the mold to
encapsulate the outer perimeter 18. As a result, conductive region
21 is formed, encapsulating the outer perimeter 18. The conductive
packaging material used is preferably thermoplastic material,
however other suitable conductive material may be used.
In the two molding processes, when the packaging material is being
injected into the mold, precautions need to be taken in preventing
the packaging material from entering the openings of the female
edge connector 10. Many prevention methods are available including
plugging the openings from inside with suitable sealant
material.
In each of the two molding processes, in order to form a solid
housing, after the injection step the packaging material may
require curing depending on the material used. This curing step may
merely include allowing the packaging material to sit in the mold
until the material hardens, or it may include a sequence of steps
of actively heating or cooling the packaging material. For
thermoset plastic material the curing step is automatic because of
the characteristic of the chemical properties of thermoset plastic.
For thermoplastic material however, it is generally advantageous to
cool thermoplastic to hasten its solidification.
After the curing step (if required), a thin layer of metallic
material 16, hereinafter referred to as a conductive label 16, is
secured to each of the top and bottom surfaces of the peripheral
card via conductive adhesive material 17. The adhesive material
used is preferably from Z-axis adhesive material, although other
suitable adhesive material may be used. Also, methods other than
use of adhesive material are available for securing the conductive
label.
The extent of the surface area covered by the conductive labels 16
is an important factor in achieving the objects of the invention.
As shown in FIG. 3, the conductive labels 16 not only cover the
entire surface area over the non-conductive regions 19 but extend
across and overlap a portion of the area over the conductive region
21. This overlap area is marked as 14 in FIG. 3.
The significance of such surface coverage by the conductive label
is twofold. First, with regions 19 covered by the conductive labels
16, the electronic components 15 are completely shielded against
any Electro-magnetic interference through the top or bottom surface
area. Second, with the conductive labels 16 overlapping the surface
area 14, electrical contact is made between the conductive labels
16 and the conductive region 21 through the conductive adhesive 17.
This electrical path is part of the discharge path for
electro-static charge built up anywhere on the surface area.
Any electro-static charge build-up on the surface of the card is
discharged through the following path: from the conductive labels
16 to the conductive region 21 via the conductive adhesive 17, and
from the conductive region 21 to the ground terminals of the edge
connector via the ground trace 12.
The ground trace 12 serves an important purpose in providing
adequate protection against both ESD and EMI. The conductive
material (region 21) covering the outer perimeter of the card is a
filled polymer which, as discussed earlier, is a poor conductor at
particular frequencies.
In the absence of the ground trace 12, ESD poses a problem in cases
where electro-static charge is built up at the end of the card
opposite the edge connector without the ground trace, the discharge
path would be through the relatively resistive region 21, across
the length of the PC card 11 to the ground terminal of the edge
connector 10. This path will not provide for an effective discharge
of the build-up charges, resulting in possible damage to the PC
board 11 or the device to which the card is coupled. With the
ground trace 12 present, the maximum distance the charge would
travel in region 21 is the distance 20 shown in FIG. 3. Since the
ground trace is highly conductive, the length of the resistive path
through region 21 is reduced to that shown by distance 20.
As discussed earlier, for effective shielding the shield must have
low enough resistivity such that no electro-magnetic energy can
penetrate. In the absence of the ground trace 12, region 21 would
not provide adequate shielding at all frequencies. The ground trace
12 embedded in region 21, improves the conductivity of this region
by providing a highly conductive parallel path to that of region
21, thereby improving the shielding characteristics of this region
against EMI.
Another aspect of this invention relates to the molding tools used
in the molding process. An important goal in any molding process is
to ensure a balanced mold flow around the PC board. To achieve this
goal, the molding tool is typically designed such that it holds the
PC board firmly in place while the packaging material is injected
into the mold. The prior art molding tool accomplishes this task by
clamping down on the active regions 13 of the PC board. This method
however, places a severe limitation on the board designers' ability
to implement any layout changes on the PC board once the molding
tool is developed. This is due to the prohibitively high
redevelopment cost of a molding tool required when the layout of
the active regions 13 is changed.
The above limitation is eliminated by the molding process of the
present invention. The first molding tool is designed such that it
clamps the outer perimeter 18 of the PC board 11. The outer
perimeter 18 contains only the ground trace 12, which is unaffected
by any layout changes in the active regions 13 of the PC board 11.
Once the active regions 13 are overmolded, forming regions 19, the
second molding tool clamps on the overmolded regions 19. Therefore,
neither of the two molding tools engage any sections of the active
regions 13 of the PC board 11. Thus, the board designer is able to
implement changes to the PC board 11 even after the molding tools
have been developed without requiring the costly step of
redeveloping a new molding tool.
Furthermore, the molding tool development can be carried out
simultaneously with the design of the PC board since the two
operations are made independent. This parallel operation results in
shorter product development and revision cycles.
In a second embodiment of the present invention, the conductive
material of region 21 is from metal which may either be
pre-fabricated or overmolded. Where the metal is overmolded, the
packaging process is similar to that of the first embodiment. Where
the metal is pre-fabricated, the pre-fabricated X is clipped on to
the PC board 11 prior to overmolding the active regions 13 of the
PC board 11. The active regions 13 can be overmolded with similar
material and in a similar manner to that of the first embodiment.
The molding tool for overmolding the active regions 13 is designed
such that it clamps the pre-fabricated metal while the packaging
material is injected into the mold. Here again the freedom to
change the board layout after the molding tool has been developed
is preserved.
As in the first embodiment, a conductive label is secured to the
top and bottom surfaces of the peripheral card. The combination of
the conductive label and the metal periphery, provide for complete
protection against ESD and EMI. In this embodiment the metal trace
is no longer needed.
Another advantage of this embodiment is the elimination of the
second molding tool and the corresponding molding steps, thereby
reducing cost and the manufacturing time cycle. Furthermore, the
metal periphery better protects against physical damage as compared
to polymer material such as thermoplastic or thermoset, and
provides a more ideal EMI shield in region 21.
An advantage associated with both of the above embodiments is that
in using a thin metal layer as the conductive label, the ESD and
EMI concerns are addressed while the thickness of the peripheral
card is kept within the thickness requirements prescribed by
industry standards.
Furthermore, the packaging techniques of the above embodiments,
lend themselves well to the chip-on-board (COB) and multi-chip
module technologies. The non-conductive material of regions 19 also
provide the protective layer normally required for the die and
wirebonds in COB technology.
The above description of embodiments of this invention is intended
to be illustrative and not limiting. The invention is further
intended to include all variations and modifications falling within
the scope of the appended claims.
* * * * *